Can I be fit even if I smoke occasionally?

You can be fitter than you are, by taking up a regular exercise programme or carrying it out more frequently or intensively. I suppose this is pretty well true anytime of anyone. You can't undo the damage that smoking does except by stopping smoking.

Even then, long term, you may find that your lungs have been affected even years and years after stopping smoking altogether. If you smoke, your lung capacity can't equal that of an identical twin who has never smoked. You smoke in the full knowledge of the harm this is doing to your lungs.